Trk family proteins are receptor tyrosine kinases composed of three family members, TrkA, TrkB and TrkC. They bind with high affinity to, and mediate the signal transduction induced by the Neurotrophin family of ligands whose prototype members are Nerve Growth Factor (NGF), Brain-Derived Neurotrophic Factor (BDNF) and Neurotrophin 3-5 (NT 3-5). In addition, a co-receptor lacking enzymatic activity, p75, has been identified which binds all neurotrophines (NTs) with low affinity and regulates neurotrophin signaling. A critical role of the Trks and their ligands during the development of the central and peripheral nervous systems have been established through gene disruption studies in mice. In particular, TrkA-NGF interaction was shown as a requirement for the survival of certain peripheral neuron populations involved in mediating pain signaling. It has been shown that increased expression of TrkA also correlates with an increased level of pain in the case of pancreatic cancer (Zhu, et al, Journal of clinical oncology, 17:2419-2428 (1999)). Increased expression of NGF and TrkA was also observed in human osteoarthritis chondrocytes (Iannone et al, Rheumatology 41:1413-1418 (2002)).
TrkA (Troponyosin-receptor kinase A) is a cell surface receptor kinase containing air extracellular, a transmembrane, and a cytoplasmic kinase domain. The binding of a neurotrophin triggers oligomerization of the receptors, phosphorylation of tyrosine residues in the kinase domain, and activation of intercellular signaling pathways, including Ras/MAPK cascade, PI3K/AKT, and 1P3- dependent Ca2+ release. Tyrosine kinase activity is an absolute requirement for signal transduction through this class of receptor. NGF receptors have been also found on a variety of cell types outside of the nervous system. For example, TrkA has been also found on human monocytes, T- and B-lymphocytes and mast cells.
There are several examples of either ant-TrkA antibodies or anti-NGF antibodies known in the art. For example, PCT Publication Nos. WO 2006/131952, WO 2005/061540 and EP 1181318 disclose use of anti-TrkA antibodies as effective analgesics in in-vivo animal models of inflammatory and neuropathic pain. PCT Application Nos. WO 01/78698, WO 2004/058184 and WO 2005/019266 disclose the use of an NGF antagonist for preventing or treating pain. PCT Application WO 2004/096122 describes a method for the treatment or the prevention of pain with co-administration of an anti-NGF antibody and an opioid analgesic. PCT Application WO 2006/137106 discloses a method for the treatment or the prevention of pain with co-administration of an anti-TrkA antibody and an opioid analgesic. In addition, profound or significantly attenuated reduction of bone pain caused by prostate cancer metastasis has been achieved by utilization of an anti-NGF antibody (Sevik, M A, et al, Pain 115:128-141 (2005)).
Besides antibodies, however, few TrkA inhibitors are known and very few (if any) show high TrkA kinase selectivity (including staurosporine derived TrkA inhibitors, CEP-751 and CEP-701). It has been rarely (if any) known in the art that a synthetic organic molecule or compound had been used as either direct TrkA or NGF inhibitor or antagonist for treatment or prevention of pain in particular. It may due mainly to the facts of difficulty in identifying potent and particularly selective anti-TrkA or anti-NGF small organic compounds, though the crystal structure of NGF in complex with the TrkA receptor has been determined (Nature 401:184-188 (1996) & 254:411 (1991)).
The therapeutic implications of an effective Trk inhibitor may well go beyond pain therapy. The subversion of this receptor and its signaling pathway in certain malignancies has also been documented. The tyrosine kinase activity of Trk is believed to promote the unregulated activation of cell proliferation machinery. It is believed that inhibitors of TrkA, TrkB, or TrkC kinases, individually or in combination, have utility against some of the most common cancers such as brain, melanoma, multiple myeloma, squamous cell, bladder, gastric, pancreatic, breast, head, neck, esophageal, prostate, colorectal, lung, renal, ovarian, gynecological, thyroid cancer, and certain type of hematological malignancies. Lestaurtinib (CEP-701, Cephalon), an indolocarbazole inhibitor of several tyrosine kinases, including Flt-3 and TrkA, and CEP-751, a pan irk inhibitor have been entered Phase II clinical trails for the treatment of acute myelogenous leukaemia (AML), pancreatic cancer and multiple myeloma (MM) and/or prostate cancer.
Of particular note are reports of aberrant expression of NGF and TrkA receptor kinase are implicated in the development and progression of human prostatic carcinoma and pancreatic ductal adrenocarcinoma and activating chromosomal rearrangements of Trks in acute myelogenous leukemia (AML), thyroid and breast cancers and receptor point mutations predicted to be constitutively activating in colon tumors. In addition to these activation mechanisms, elevated Trk receptor and ligand have also been reported in a variety of tumor types including multiple myeloma, melanoma, neuroblastoma, ovarian and pancreatic carcinoma. The neurotrophins and their corresponding Trk receptor subtypes have been shown to exert a variety of pleiotropic responses on malignant cells, including enhanced tumor invasiveness and chemotaxis, activation of apoptosis. stimulation of clonal growth, and altered cell morphology. These effects have been observed in carcinomas of the prostate, breast, thyroid, colon, malignant melanomas, lung carcinomas, glioblastomas, pancreatic carcinoids and a wide variety of pediatric and neuroectodermal-derived tumors including Wilm's tumor, neuroblastomas and medulloblastomas. Neurotrophins and their receptor subtypes have been implicated in these cancers either through autocrine or paracrine mechanisms involving carcinoma cells and the surrounding parenchymal and stromal tissues. Overall, the oncogenic properties of Trk signaling in multiple tumor types makes the modulation of the Trk receptor signaling a potentially attractive therapeutic intervention point in different malignancies.
Due to the therapeutic promise associated with inhibiting TrkA, and the relative lack of potent and selective inhibitors, it is great need to discover the potent and particular isoform selective TrkA inhibitors, especially of orally active small synthetic molecules for possible treatment or prevention of the disease or disorders associated with TrkA activity.